1. Field of the Invention
The present invention relates to an electrophotographic photoreceptor, and to an image forming apparatus and a process cartridge using the electrophotographic photoreceptor.
2. Discussion of the Background
Recently, development of information processing systems utilizing electrophotography is remarkable. In particular, optical printers in which information converted to digital signals is recorded using light have been dramatically improved in print qualities and reliability. This digital recording technique is applied not only to printers but also to copiers, and so-called digital copiers have been developed and used. Copiers utilizing both the conventional analogue recording technique and this digital recording technique have various information processing functions, and therefore it is expected that demand for such copiers will be escalating. In addition, with popularization and improvement of personal computers, the performance of digital color printers which can produce documents.
Electrophotographic photoreceptors for use in these image forming apparatuses are broadly classified into organic photoreceptors and inorganic photoreceptors. Recently, the organic photoreceptors are widely used because of having advantages of being produced more easily with less cost, able to use more various materials such as charge transport materials, charge generation materials and binder resins, and more freely designed than the inorganic photoreceptors.
The organic photoreceptors include a single-layered photoreceptor wherein a charge transport material (CTM) including a positive-hole transport material and an electron transport material, and a charge generation material (CGM) are dispersed together in a same photosensitive layer; and a multilayered photoreceptor wherein a charge generation layer (CGL) including a CGM and a charge transport layer (CTL) including a CTM.
Most of the multilayered photoreceptors are negatively charged, and positively-charged multilayered photoreceptors are not in practical use. This is because CTMs having good charge transportability, less toxicity and high compatibility with a binder resin are not in practical use.
However, the negatively-charged photoreceptor is not stably charged with a corona charger and produces ozone or NOx, which are absorbed to the surface of the photoreceptor, resulting in physical and chemical, and further environmental deterioration. Therefore, the positively-charged photoreceptor having more flexible use conditions is more advantageously used.
The positively-charged photoreceptors include single-layered photoreceptors. Most of the single-layered photoreceptors include both of the positive-hole transport material and the electron transport materials as a charge transport material, and therefore the single-layered photoreceptor has a sensitivity having both positive and negative polarity. However, most of the single-layered photoreceptors are positively charged because of having better sensitivity and taking advantage of the merits of being positively charged.
Japanese Published Unexamined Patent Applications Nos. 8-328275, 7-64301, 9-281729, 6-130688 and 9-151157 disclose single-layered organic photoreceptors, which have higher residual potential, and larger variation of potential due to repeated use and after irradiated than functionally-separated multilayered photoreceptors.
In order to solve such problems of the single-layered photoreceptors, new electron CTMs are now being developed. A tetracarboxylic acid and a naphthalenecarboxylic acid disclosed in WO2005092901 having good charge transportability can solve the problems and largely improve the electrostatic characteristics of the single-layered photoreceptors.
Further, the image forming apparatus is required to downsize and print at higher speed recently, and the photoreceptor is required to have high sensitivity. Latest digital image forming apparatuses typically use a laser diode (LD) and a light emitting diode (LED) mostly having a near-infrared area wavelength of from 680 to 830 nm. Therefore, electrophotographic photoreceptors using phthalocyanines, particularly a titanylphthalocyanine (TiOPc) having high sensitivity to the near-infrared area as a CGM are actively developed.
Various crystal forms of the titanylphthalocyanine are known, and particularly a titanylphthalocyanine crystal having a CuKα 1.542 Å X-ray diffraction spectrum comprising a maximum diffraction peak at a Bragg (2θ) angle of 27.2° disclosed in Japanese Published Unexamined Patent Applications Nos. 2001-19871, 11-5919 and 3-269064 is known to have very high carrier generability.
Multilayered electrophotographic photoreceptors using this titanylphthalocyanine crystal are in practical use, but single-layered ones are not good. This is because the titanylphthalocyanine crystal deteriorates the chargeability of photoreceptors, particularly single-layered ones.
A single-layered photoreceptor including a CTM for use in the present invention in a range disclosed in WO2005092901 and a titanylphthalocyanine as a CGM has very high sensitivity. However, the titanylphthalocyanine deteriorates the chargeability, and potential of the single-layered photoreceptor due to repeated use, resulting in abnormal images such as background fouling.
In addition, the single-layered photoreceptor tends to produce residual images, and more when using the titanylphthalocyanine as a CGM
Because of these reasons, a need exists for a single-layered photoreceptor having high sensitivity, being stably charged and not producing abnormal images such as residual images even after repeatedly used.